The present invention relates to a method and apparatus for applying a layer of coating material to a substrate and, more particularly, to a method and apparatus for applying a thin, virtually constant thickness coating to a substrate.
In many coating applications, extremely thin, constant thickness coatings are absolutely essential in order to avoid degrading the performance of the coated device and/or the equipment with which such coatings are utilized. In, for example, audio and video magnetic tapes, if the magnetic media coating is excessively thick or there are significant variations in coating thickness, magnetic coupling and therefore information transferal between the magnetic media in the coating and, for example, a read/write head of audio or video recording or reproducing equipment in which the tape is utilized could be substantially degraded because of the increased spacing or the spacing variations between these components that necessarily result when such coatings are employed.
A number of coating techniques presently exist for applying coating materials to a web or other object surface. Many of these techniques employ an electrostatic field between the coating applicator and the web or object surface to assist in both the uniform and efficient deposition of coating materials on such surfaces. In, for example, the well-known process of electrostatic spray painting, an electrostatic field is established between an electrically conductive grid and a particular metal object to be painted. The electrostatic field is created by a relatively high dc voltage (100,000 V) connected between grid and object, with the object ordinarily being spaced several feet from the grid. Air pressure supplied to a reservoir of coating fluid coupled to one or more orifices in the coating applicator causes coating fluid droplet formation at the output of each of said coating applicator orifices. The droplets are subsequently propelled into the grid by air pressure generated forces where they become electrostatically charged and then deposited, in layer form, on a surface of said particular object by forces associated with the electrostatic field. Unfortunately, due to the relatively large droplet size generated by this type of coating apparatus, the resulting coating layers are well in excess of a thickness level that would avoid the above-mentioned problem associated with excessively thick magnetic media coatings.
Electrostatic coating apparatus capable of generating and subsequently depositing relatively small coating material particles on a substrate has been described in the patent literature. In W. A. Starkey et al U.S. Pat. No. 2,685,536, for example, a method and apparatus for electrostatically coating articles are disclosed wherein coating material is supplied to the orifice of a coating applicator and an electrostatic field is established between the surface of the coating material and a surface of the article to be coated. As described therein, the coating material oozes through applicator orifices where it is then divided into fine particles by forces associated with the electrostatic field. The finely divided particles are then transported to the surface of the article by forces associated with the same electrostatic field.
It can be demonstrated that the rate of electrostatic field generated, coating material particle movement toward a surface to be coated in, for example, the above-noted STARKEY ET AL apparatus, is dependent, in part, upon the intensity of the electrostatic field. Therefore, variations in electrostatic field intensity when coating, for example, a moving web will produce corresponding variations in the thickness of an electrostatically deposited coating layer. It is well known that the intensity of an electrostatic field established between a pair of electrodes spaced a fixed distance from one another is very much dependent upon the shape of the electrode ends between which the electric field is established. The smaller the radius of curvature at the end of an electrode, for example, the greater will be the electrostatic field intensity in the vicinity of the small radius of curvature electrode for any particular electrode-to-electrode voltage. Variations in electrode shape, therefore, can produce corresponding variations in an electrostatic field in the vicinity of the electrode surface where the shape is varying.
If, for example, the external surface of coating material oozing through an applicator opening in FIG. 2 of STARKEY ET AL, is considered to be an end of an electrically conductive electrostatic field generating electrode, variations in the shape of this external coating material will produce variation in an electrostatic field in the vicinity of said coating material surface and corresponding variations in the thickness of coating material being atomized and deposited on a surface by such a varying electrostatic field. Such surface shape variations can be caused by any number of different factors. One factor is the pressure variations that are necessarily produced when a mechanical pump is is employed to pressurize coating material supplied to the coating applicator. Another factor could be the presence of air bubbles entrained within the coating material. These bubbles would momentarily disturb or change the external surface shape of the coating material when passing through an applicator orifice. Yet another factor might be changes in the coating surface shape that may result as coating material particles are electrostatically extracted from said coating surface during the coating process. Each of these electrostatic field intensity altering factors can produce variations in web coating layer thickness.
Also, the electrostatic coating apparatus described in STARKEY ET AL is employed where the object to be coated is itself electrically conductive. In such an arrangement, the ability to coat a particular object is primarily dependent upon the ability to establish an electrostatic field of sufficient intensity between the coating applicator and a surface of the object to be coated to both atomize the coating material particles and then transport same to said object surface. However, this type of coating apparatus is relatively ineffective when employed to coat insulative materials, such as a dielectric web, because the electrostatic field needed to coat such materials is significantly attenuated by the dielectric web. Present practice is to overcome this problem by coating the web with an electrode forming layer of electrically conductive material and then electrostatically coat this layer with the preferred or final layer of coating material. Unfortunately, this technique both increases the cost of coating a web and adds the characteristics of the conductive layer to that of the coated web which may prevent its subsequent use in many coated web applications.
In addition, it can readily be demonstrated that fluids are dried by or have moisture removed from them when exposed to an electrostatic field. If coating material flow rates are in the near-zero flow rate range, as in the STARKEY ET AL patent, the external surface of the coating material at the coating applicator orifices or outputs may dry out or coagulate due to the presence of an electrostatic field at said coating material surfaces and thereby block or substantially reduce the flow of coating material through each coating applicator orifice, within a relatively short period of time. A flow rate change of this type would also produce detrimental variations in web coating layer thickness.
It is a primary object of the present invention, therefore, to provide coating apparatus that is capable of applying a relatively thin, virtually constant thickness layer of coating material to a substrate.
It is another object of the present invention to provide extremely low flow rate coating apparatus having a flow rate that will remain fairly constant for an extended period of time.
It is a further object of the present invention to provide low flow rate coating apparatus that is capable of properly functioning within an electrostatic field for an extended period of time.
An advantage of the present invention is the ability of the coating apparatus employed therein to apply coating materials having significantly greater conductivity than that of coating materials presently applied by conventional electrostatic coating techniques.
Other objects, features and advantages of the present invention will be readily apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.